Amyotrophic lateral sclerosis (ALS) is a progressive and untreatable neurodegenerative disease that is characterized by the selective death of upper and lower motor neurons (MNs). The overwhelming majority of the disease is sporadic in nature. However a relatively small (<12%) but highly informative fraction of patients suffer from familial forms of disease, which have enabled the identification of causative genetic variants that underlie their condition. Such genetic studies have demonstrated that ALS can be caused by mutations in genes that encode proteins involved in diverse set of cellular functions ranging from RNA processing, vesicle transport, cytoskeletal regulation, mitochondrial function, and protein quality control pathways. Nevertheless, ALS patients are uniformly characterized by a common pattern of progressive motor neurodegeneration. This raises the possibility that different disease initiating events could coalesce in one or more common molecular pathways. How the mutation of genes with dissimilar functions converge on MN degeneration has been and continues to be an outstanding question. Although all ALS patients exhibit neuropathological protein aggregates, the overall contribution of protein homeostasis in causing ALS has remained unclear. If we could identify a convergent mechanism, it may provide an opportunity to develop a broadly applicable therapeutic intervention strategy. In our preliminary studies, we conducted global analysis of protein degradation dynamics in mutant SOD1 and isogenic controls MNs derived from iPSC lines. Interestingly, we identified a number of proteins that are degraded at a slower rate in SOD1 MNs. Unexpectedly, this small panel of candidates included proteins whose genetic mutations cause ALS. In the proposed research we will use patient-derived neurons coupled with mass spectrometry analysis to determine the protein substrates, as well as the nature of the perturbation that arise as a result of mutations in the two most prevalent ALS genes: SOD1 and C9orf72. First, we will determine which proteins have reduced protein degradation dynamics. Second, we will determine which proteins have altered synthesis rates. Third, we will determine the overall degree of proteome-wide remodeling. Each of these approaches has strategic advantages over traditional work-flows and will allow us to determine not only which proteins have altered levels in ALS MNs but also the mechanism responsible for their perturbation. Taken together, our proposed aims will shed light into the cellular mechanisms compromised by changes in the proteostasis network in patient neurons and will likely uncover broadly relevant therapeutic targets for ALS.